Pressurized-water nuclear reactors have a core formed of prism-shaped assemblies arranged vertically and resting on a support plate within the vessel of the nuclear reactor.
During the operation of the nuclear reactor, it is necessary periodically to carry out flux measurements within the actual interior of the core. For this purpose, very small fission detectors are used, and these are displaced by remote control, by means of teleflex cables, within tubes closed at one of their ends and called glove fingers. The glove fingers are introduced according to a predetermined distribution into the entire height of some core assemblies after passing inside instrumentation tubes. As a result of the displacement of the flux detectors within the glove fingers introduced into the assemblies, flux measurements can be carried out over the entire height of the core. It must be possible to extract the glove fingers from the core assemblies, for example to make it easier to carry out the operations of refuelling the reactor core; for this purpose, a pull is exerted on the end of the glove fingers from an instrumentation room located laterally in relation to the well of the reactor vessel. The glove fingers are arranged in guide tubes, one of the ends of which opens into the instrumentation room and the other end of which opens into the inner volume of the vessel by means of a vertical sleeve passing through the convex vessel bottom. Between these two ends, the guide tube has a bent part with a relatively large radius of curvature.
The lower internal equipment of the reactor comprises, in addition to the plate which supports the core and on which the assemblies rest, the shell and the partitioning of the core and a set of elements arranged between the core support plate and the lower convex bottom of the vessel and comprising, in particular, hollow instrumentation guide columns, the bore of which is an extension of the bore of the guide-tube passage sleeves. The central duct of these guide columns opens onto the upper face of the core support plate in alignment with an instrumentation tube of an assembly resting on the support plate.
The inside diameter of the guide tubes, the guide columns and the instrumentation tubes of the assemblies is such that there is sufficient play between the glove finger and its guide duct. It is therefore easy to maneuver the glove fingers by pulling and pushing them respectively to extract them from the core over the entire height of the latter (i.e., over a length in the region of 4 metres) and to reintroduce them into the core.
However, it was noted that, after the reactor had been in operation for a certain length of time, the force needed to extract the glove fingers, and above all the force needed to reinsert them, increased substantially.
For example, the glove-finger insertion force which is 100 to 150 newtons on a reactor at the time of commissioning increases to a value of 400 to 500 newtons after the second refuelling. This increase in the glove-finger insertion force could be attributed to the presence of solid particles which were deposited between the glove finger and the inner wall of the guide tube, especially in the vicinity of the bend of this guide tube. In fact, the glove fingers, during their displacement, cause an accumulation of particles in the region of the bend and make it easier for them to settle.
A process for cleaning the guide tubes at the time of the operations of refuelling the reactor core was therefore proposed. In this case, the vessel is open in its upper part and in communication with the reactor pool, the assembly as a whole being filled with water. To carry out the cleaning process, means are provided for injecting demineralized water under pressure into the guide tubes via their end opening into the instrumentation room.
However, it is necessary to recover this demineralized water and the particles which it contains in suspension at the other end of the guide tubes.
In nuclear reactors in operation at the present time, there are 50 guide tubes passing through the convex bottom of the vessel by means of 50 sleeves distributed uniformly over this convex bottom and arranged underneath the lower internal equipment of the reactor. The sleeves themselves open with substantial play into the bore of the lower part of the guide columns forming part of the lower internal equipment.
It is therefore necessary to recover the demineralized cleaning water, containing the particles which are radioactive, at the outlet of the passage sleeves. To gain access to these sleeves, the lower internal equipment is therefore removed and extracted from the vessel, the upper internal equipment and the core assemblies having been extracted beforehand.
Means of recovering the water and the radioactive particles are brought sucessively into place on the end of each of the sleeves during the injection of pressurized water into the corresponding guide tube.
These means are put in place from the platform of the fuelling machine, above the reactor pool, by means of a very long pole which carries in its lower part a camera making it possible to display the operations of installing the recovery means. These means comprise a cylindrical cap provided on the inside with a gasket of a diameter corresponding to the diameter of the sleeve, a flexible tube which communicates with the inner volume of the cylindrical cap and on which is located a valve controlled by a linkage from the platform, and a filtration unit at which the flexible tube terminates above the level of the reactor pool.
It is difficult to carry out these operations, the control of which via a camera fixed to the lower end of the pole can only be obtained under very good conditions.
Furthermore, to carry out this process, it is necessary to extract the lower internal equipment from the vessel and therefore to reinstall it after the operation. The operation of extracting and the operation of reinstalling the lower internal equipment creates considerable risks.